The invention relates to a device for distinguishing good from bad among flat small workpieces, and more particularly a device capable of high-precision inline inspection of coins formed e.g. by a press.
High accuracy is sometimes required for thin, flat, small workpieces formed by continuously blanking a sheet material in a press. To remove defective workpieces, it is necessary to examine each workpiece for its dimension, chipping of the edge, and position of holes or slits if any.
An image sensor (or image processor) is considered to be the best choice for such examination. A typical such image sensor includes a high-definition CCD camera for photographing each workpiece, and a control unit for subjecting the photos taken to binarizing and density adjustment and operation to determine if each workpiece is up to standard. This sensor is used for examination on whether or not necessary parts are actually mounted, and for size, directional, dimensional, positional and other examinations. Newest image sensors even have sub-pixel processing functions, i.e. the function of processing images on the order of 0.1 pixel, and the function of counting the number of edges. Such sensors increase the reliability of examination.
Some workpieces formed by blanking in a press sometimes require certain post-treatment. For example, on a press, a small amount of oil is applied to a mold of a press to extend the life of the mold and to improve releasability. In such a case, it is necessary to remove oil from workpieces formed by the press by washing and then to dry them. It is desired to pick out and eliminate any defective workpieces before such post-treatment to save time, cost and energy.
Finding defective workpieces as soon as possible is also important in finding any failure situation of the press. For example, from the fact that defective workpieces are found continuously, one can know that the mold of the press has been worn or otherwise damaged.
For this reason, inline examination of workpieces immediately after they are formed by pressing is extremely effective. But actually, it is difficult to examine workpieces while being carried on a belt conveyor from the press with high reliability.
Flat small workpieces are laid flat on the belt conveyor, and are photographed by an overhead camera while shedding light from above (reflection lighting) for examination by an image sensor.
One problem of such a reflection lighting system is that the camera cannot distinguish soiling, scratches and foreign matter on the conveyor from deficiencies of workpieces.
The degree of soiling of the conveyor varies with the number of workpieces processed and the degree of oil adhered. But the camera usually tends to pick up noises (on the images) in less than several hours.
If the belt conveyor is made of a soft material such as polyurethane, foreign objects such as powdery metal pieces tend to stick on the belt surface. Once stuck, they are not easily removable by e.g. a scraper. The number of such foreign objects and scratches on the belt surface tends to increase with time, and they appear as noises in the photos taken by the camera. When such noises increases to a certain level, no accurate examination is possible any longer.
When workpieces are formed by blanking on a press, they are blanked from a front side of the sheet to its back side. Thus, the edges of the front and back sides of each workpiece have slightly different shapes from each other. Thus, for accurate examination of such blanked workpieces, it is necessary to lay them on the conveyor belt so that the front sides of all the workpieces face the same direction. Also, even if the workpieces are laid with their front sides facing all up or all down, edge shapes also tend to change with time because the model is worn gradually. As the edge shapes change, light reflectance changes, thus affecting the accuracy of examination.
An object of the invention is to provide a device which can examine workpieces with high accuracy.
According to the invention, there is provided a device for examining flat workpieces having front and back sides, the device comprising a turntable having a workpiece-carrying portion formed from a transparent hard material, a conveyor for carrying the workpieces with front or back side thereof in contact with the conveyor surface, a transfer mechanism for moving the workpieces being carried on the conveyor onto the workpiece-carrying portion of the turntable, a guide for guiding the workpieces on the turntable to an examination point on the turntable while the turntable is turning, a camera for photographing the workpieces one by one when they reach the examination point, an image sensor unit for determining whether each of the workpieces photographed by the camera is defective or nondefective, and a workpiece discharge unit for discharging the defective and non-defective workpieces out of the turntable separately from each other, the image sensor unit including a lighting unit provided opposite the camera with the turntable therebetween for shedding light on each workpiece at the examination point through the turntable.
According to this invention, the lighting unit is a strobe operatively associated with the camera so as to flash when the camera is activated.
According to this invention, an oil film is formed between the workpieces and the turntable to prevent the workpieces from moving relative to the table.
According to this invention, a scraper for scraping off foreign matter or oil on the turntable is provided.
Works are examined on a turntable, not on the conveyor. Thus, any soiling, scratches or foreign matter on the conveyor will have no influence whatsoever on the accuracy of examination.
The transmission lighting arrangement allows the camera to clearly photograph works because works, which do not pass light, contrast clearly against the background, i.e. turntable, which is transparent and thus passes light, and because no reflected light enters the camera.
The hard turntable is practically scratch-free, and any soiling or foreign matter can be easily wiped off. Thus its surface can be kept clean for a long time.
The strobe lighting allows the camera to catch more vivid images of works.
Oil film formed on the surface of works serves as a kind of adhesive for keeping the works firmly stuck on the turntable. This makes it possible to turn the table at a higher speed and thus to examine a greater number of works per unit time.
Other features and objects of the present invention will become apparent from the following description made with reference to the accompanying drawings, in which: